Adolf Hitler decided to visit Finland on June 4, 1942, ostensibly to congratulate Mannerheim on his 75th birthday. But Mannerheim did not want to meet him in his headquarters in Mikkeli nor in Helsinki, as it would have seemed like an official state visit. The meeting took place at a railway siding near the airfield at Immola, in south-eastern Finland, and was arranged in secrecy.
During the visit, an engineer of the Finnish broadcasting company YLE, Thor Damen, succeeded in recording Hitler's and Mannerheim's private conversation. This had to be done secretly, as Hitler never allowed others to record him off-guard. This is the only known recording of Hitler speaking in an unofficial tone.